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EFFECT OF REVERSIBLE CROSSLINKS ON NANOSTRUCTURE AND PROPERTIES OF SUPRAMOLECULAR HYDROGELS
Wang, Chao

2018, Doctor of Philosophy, University of Akron, Polymer Engineering.
Hydrogels are water-swollen polymer networks that often exhibit biocompatibility and mechanical properties similar to many natural tissues. These properties grant hydrogels biomedical applications, such as tissue engineering, wound dressing and drug delivery. In recent years, supramolecular hydrogels with reversible, non-covalent crosslinks have received attention by researchers because of their toughness and the ability to be molded into complex shapes by conventional polymer processing techniques. During deformation, the reversible crosslinks (hydrophobic associations, ionic bonds, and/or hydrogen bondings) will dissociate and re-form to dissipate mechanical energy and prevent failure. In order to design tough supramolecular hydrogels, it is important to understand their structure-property relationships, i.e., the effect of crosslink structural change on the mechanical/rheological properties of hydrogels.

This work investigated physically crosslinked hydrogels composed of random copolymers with hydrophilic segments [N,N-dimethylacrylamide (DMA) or 2-hydroxyethyl acrylate (HEA)] and hydrophobic segments [2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate/methacrylate (FOSA or FOSM)]. When swollen in water, these copolymers form networks by the hydrophobic aggregation of FOSA/FOSM segments into nanodomains. The nanostructure of these hydrogels was elucidated by small angle neutron scattering (SANS) with contrast variation. The in-situ nanostructure evolution of the hydrogels during deformation and relaxation was monitored by small angle X-ray scattering (SAXS) and correlated with the mechanical properties of the hydrogels. During uniaxial extension, the rearrangement of crosslinks altered the network conformation and resulted in a non-affine deformation. With increasing strain rate, more rearrangement of crosslinks occurred to dissipate mechanical energy and toughen the hydrogel. At the same time, more structural anisotropy occurred due to less network relaxations.

Due to the aforementioned structure-property relationships of supramolecular hydrogels, the mechanical/rheological properties can be modulated by modifying the nanostructure, so as to suit the need in different applications. In this dissertation, three methods for modifying the nanostructure and mechanical/rheological properties of supramolecular hydrogels are discussed: surfactant addition, thermal annealing, and incorporation of a secondary physical crosslink. The incorporation of a secondary, stronger ionic crosslink slows down the network relaxation. On the other hand, addition of surfactant or heating increase the mobility of the hydrophobic nanodomains and lead to conformational rearrangement of network chains to reduce the network defects. This would increase the crosslink density of the network and strengthen the hydrogel.

During crystallization, the volume expansion of water can be destructive. This is not desirable in applications such as cryogenic preservation, food preservation and agriculture. A family of hydrophobically crosslinked hydrogels is designed to suppress ice formation. Combining two methods from Nature: hydrogen bonding of water to the hydrophilic groups in the hydrogel and confinement of water by the small spacings between hydrophobic crosslinks, nearly 100% of water crystallization is inhibited at temperatures as low as 203 K.

In summary, the structure-property relationship of supramolecular hydrogels during deformation and relaxation was elucidated. The mechanical/rheological properties of these hydrogels were modulated by surfactant addition, thermal annealing or incorporation of secondary crosslinks. Taking the merit of hydrogen bondings in the hydrogel and the small spacings between hydrophobic associations, a family of hydrogels that can supercool their absorbed water at cryogenic temperatures were designed.
Bryan Vogt (Advisor)
Robert Weiss (Advisor)
Abraham Joy (Committee Member)
Mark Soucek (Committee Chair)
Qixin Zhou (Committee Member)
256 p.

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Wang, C. (2018). EFFECT OF REVERSIBLE CROSSLINKS ON NANOSTRUCTURE AND PROPERTIES OF SUPRAMOLECULAR HYDROGELS. (Electronic Thesis or Dissertation). Retrieved from https://etd.ohiolink.edu/

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Wang, Chao. "EFFECT OF REVERSIBLE CROSSLINKS ON NANOSTRUCTURE AND PROPERTIES OF SUPRAMOLECULAR HYDROGELS." Electronic Thesis or Dissertation. University of Akron, 2018. OhioLINK Electronic Theses and Dissertations Center. 22 Oct 2018.

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Wang, Chao "EFFECT OF REVERSIBLE CROSSLINKS ON NANOSTRUCTURE AND PROPERTIES OF SUPRAMOLECULAR HYDROGELS." Electronic Thesis or Dissertation. University of Akron, 2018. https://etd.ohiolink.edu/

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